Modern gas turbine engines operate in high temperature environment in excess of 2000° F. in which hot gases are expanded across rows of turbine blades. Outer air seal or shroud segments circumscribe the turbine blades to minimize leakage of the gases over the tips of the blades. The use of thermal barrier coatings on gas turbine blades and surfaces such as shroud segments have been found to have several advantages. Through the use of thermal barrier coatings higher operating efficiency can be obtained because less cooling air is required to maintain blade or shroud temperatures. In addition, component life is extended since the rate of change of metal temperature is reduced by the insulating effect of the thermal barrier.
Zirconia based thermal barrier coatings, because of their low thermal conductivity, are added to the surface of metal components to insulate them from the hot gas stream. Stabilized zirconia was developed and used as a thermal barrier coating for turbine and shroud components. Coatings such as CaO stabilized zirconia, MgO stabilized zirconia and yttria stabilized zirconia have been tested with yttria partially stabilized zirconia providing the best results.
U.S. Pat. No. 5,073,433 discloses a thermal barrier coating for substrates comprising zirconia partially stabilized by yttria and having a density of greater than 88% of the theoretical density with a plurality of vertical macrocracks homogeneously dispersed throughout the coating to improve its thermal fatigue resistance. This patent also discloses a process for producing the thermal barrier coating.
The continues to be a need in the art to provide improved thermal barrier coating for components intended to be used in cyclic thermal environments such as for gas turbine blades, vanes and seal surfaces exposed in the hot section of gas turbine engines.